Pavlovian conditioning and extinction of context cues and punctate CSs in rats with ibotenate lesions of the hippocampus

The degree to which conditioned stimuli (CSs) control responding is often i nfluenced when contextual cues are also associated with the unconditioned s timulus (US). In Experiment I, selective ibotenate (IBO) lesions of the hip pocampus impaired the ability of rats to inhibit responding to contextual c ues made excitatory by presentation of unsignaled USs during simple Pavlovi an CS-US training. Hippocampal damage had little effect on responding if fe w or no unsignaled USs had been delivered in the context. In Experiment 2, there was no evidence of increased responding to contextual cues when IBO l esions were given after excitatory training. However, lesioned rats showed greater resistance to CS extinction than did controls. An interpretation th at encompasses these and other findings is that selective lesions of the hi ppocampus impair the formation of at least some types of inhibitory associa tions.